1. Field of the Invention
The present invention relates to an apparatus and method for measuring the angle between two members, such as in measuring the toe angle of a vehicle wheel. More particularly, the present invention relates to an instrument and method which provides a direct measurement of the angle between two members to give a reading without reliance on the measurement of other parameters.
2. Description of the Prior Art
One method used in adjusting the toe angle of the front wheels of a vehicle is based on deriving the toe angle from a known distance to a screen and a given displacement of a projected beam on the screen, as shown, for example, in U.S. Pat. No. 3,953,134. A more direct method of measuring the toe angle utilizes the projection of a scale to a target screen which has a reference line thereon. In a system of this type, the distance of the screen from the projector (which is mounted on the wheel) is not critical because the image will be larger on a more remote screen and smaller on a near screen. In other words, although the spacing between two indicia on the scale changes with distance of the scale from the projector, the angle formed by lines extending from the projector to the indicia does not change. The movement of the projector beam, due to adjustment of the toe of the wheel, from one indicia to the other, will be a movement through the same angle regardless of the precise distance of the projector to the screen.
The problem with scales on screens, whether projected or not, stems from the difficulty of reading the scale accurately. Frequently, the operator is a considerable distance from the screen, and ambient light makes the task more difficult. It is therefore generally preferable to employ an electronic readout of the projected beam and target. Some electronic toe reading apparatus, such as shown in U.S. Pat. No. 3,865,492, uses a fixed target, as distinguished from a projected target, and the electronic readout must be calibrated in accordance with the distance between the projector and the target.
Another system employing an electronic readout of wheel toe is shown in the U.S. Pat. No. 3,782,831. In this system, a unit which is mounted to the wheel to be measured has a projector thereon. The projector is swung back and forth to sweep a beam across a detector which is spaced from the unit. An instantaneous analog voltage is developed during the sweep of the projector which is proportional to the angle of the beam at any given instant. When the beam strikes the detector, a readout is obtained which gives the toe of the wheel on which the unit is mounted.